The present invention relates to a light-emitting device comprising a gallium-nitride-group compound semiconductor, used in opto-devices such as a light-emitting diode, a laser diode, etc. More specifically, a semiconductor light-emitting device, with which the operating voltage can be lowered, at the same time, the efficiency of light emission can also be improved.
Gallium-nitride-group compound semiconductors have been increasingly used as semiconductor material for the visible light-emitting devices and the electronic devices of high operating temperature. The development has been significant in the fields of, in particular, blue light-emitting diodes, green light-emitting diodes and blue-purple laser diodes.
A basic method of manufacturing the light-emitting device comprising the gallium-nitride-group compound semiconductor is growing a gallium-nitride-group semiconductor film on the surface of a substrate of sapphire, SiC, etc. by means of metal organic chemical vapor deposition. In a practical process of forming a compound semiconductor layer of gallium-nitride-group, a substrate is placed in a reaction tube; metal organic compound gases, for example tri-methyl-gallium (TMG), tri-methyl-aluminum (TMA), tri-methyl-indium (TMI), as the material gas for the Group III element, and ammonia, hydrazine, etc. as the material gas for the Group V element, are supplied therein. The substrate is maintained at a high temperature 900.degree. C.-1100.degree. C., and an n-type layer, a light-emitting layer and a p-type layer are grown on the substrate in a stacked layer structure. After that, by using the technologies of photolithography, vapor deposition, etc., an n-side electrode and a p-side electrode are formed, respectively, on the n-type layer and the p-type layer. Thus, a light-emitting device is completed.
In the semiconductor light-emitting devices, including those of the gallium-nitride-group compound-semiconductor, a material for forming the light-emitting layer is selected in accordance with a requested wavelength of light-emission. The electron injection from the n-type layer to light-emitting layer and the hole injection from the p-type layer into the light-emitting layer cause a recommbination of the electron and the hole within the light-emitting layer to generate a light of a certain desired wavelength.
Among the gallium-nitride-group compound semiconductor light-emitting devices, those having a double hetero junction structure comprising an n-type clad layer of GaN, AlGaN, a light-emitting layer of InGaN and a p-type clad layer of AlGaN are forming the main current of products. FIG. 5 shows the structure of a conventional gallium-nitride-group compound semiconductor light-emitting device.
As shown in FIG. 5, an n-type layer 13 of gallium-nitride (GaN), a light-emitting layer 14 of indium-gallium-nitride (InGaN) and a p-type clad layer 15 of aluminum-gallium-nitride (AlGaN) are stacked on a sapphire substrate 11, with the intermediary of a buffer layer 12, to form double hetero junction structure. Stacked over it is a p-type contact layer 16 of GaN A p-side electrode 17 is formed on the p-type contact layer 16, and an n-side electrode 18 is formed on the surface of the n-type layer 13, which has been exposed as a result of the removal in part of the three layers, viz. p-type contact layer 16, p-type clad layer 15 and light-emitting layer 14.
In the above described gallium-nitride-group compound semiconductor light-emitting device, when a voltage of negative polarity is applied on the n-side electrode 18 and a voltage of positive polarity is applied on the p-side electrode 17, electron is injected from the n-type layer 13 into the light- emitting layer 14, at the same time the hole is injected from the p-type clad layer 15 into the light-emitting layer 14. Thus, the light-emitting layer 14 emits a light having an energy corresponding to the band gap energy of the semiconductor material constituting the light-emitting layer 14. The above structure has been disclosed by, for example, Japanese Laid-open Patent No 6-268259.
The structure containing the double hetero junction provides a significant improvement in the output of the light emission and in the operating voltage, as compared with a conventional light-emitting device of metal - insulator - semiconductor (MIS) structure.
With the improved light-emitting output and the operating voltage, the light-emitting display devices comprising gallium-nitride-group compound semiconductor have now become usable in, for example, the outdoor display application.
However, in the large-size display devices, including those for outdoor use, the clear visibility is readily affected by the strong mid-day sunlight. Therefore, a further increase in the light-emitting output is requested in order to present a better image of higher visibility. Also with the view to curtailing the power consumption of the large outdoor display devices, reduction in the operating voltage of a light-emitting device is asked for.
It is known that the higher the injection efficiency of electron into the light-emitting layer the higher the output of light emission of a light-emitting device; suppressing the overflow of electron into the p-type layer is an effective measure to increase the efficiency of electron injection into the light-emitting layer. In order to suppress the electron overflow into the p-type layer, it is necessary to raise the energy barrier in the conduction band of the p-type clad layer forming the double hetero junction structure. In a gallium-nitride-group compound semiconductor light-emitting device as shown in FIG. 5, for example, the energy barrier in the conduction band can be raised by increasing the Al composition in the p-type clad layer 15 formed of AlGaN.
However, if the rate of Al composition in the p-type clad layer 15 is increased, the energy barrier against hole at valence band in the junction between the p-type clad layer 15 and the p-type contact layer 16 formed of GaN tends to go higher; which can be understood easily from the band structure shown in FIG. 6. When the energy barrier against hole in the junction between p-type clad layer 15 and p-type contact layer 16 is high, an excessive voltage-drop is caused to lower the energy barrier when a driving voltage is applied to obtain a light emission. Accordingly, the operating voltage of the light-emitting device becomes high. The present invention addresses the problem, and aims to form a gallium-nitride-group compound-semiconductor light-emitting device that provides a lowered operating voltage and an increased light emission output altogether. Using such light-emitting devices enables to offer an outdoor display device of a superior visibility.